AI: Wrap SDK constructor exceptions into WP_Error in prompt builder

[gziolo]

Trac ticket: https://core.trac.wordpress.org/ticket/64591

Summary

• Wraps the SDK's PromptBuilder constructor call in a try/catch inside WP_AI_Client_Prompt_Builder::__construct(), so invalid input (empty string, empty array, invalid type) is stored as a WP_Error instead of throwing an InvalidArgumentException
• The error surfaces consistently through the fluent chain when a generating method is called, matching the existing __call() behavior
• Updates three existing tests to assert WP_Error directly instead of catching exceptions
• Adds a new test verifying wp_ai_client_prompt( ' ' ) returns a builder that yields WP_Error from generate_text()

Follow-up to #10881 (comment).

Test plan

• Verify npm run test:php -- --group ai-client passes (209 tests)

🤖 Generated with Claude Code

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Props gziolo, flixos90.

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[felixarntz]

@gziolo Thanks, LGTM! Just a tiny nit.

[felixarntz]

Committed in https://core.trac.wordpress.org/changeset/61787